ActiveSupport dependency

`Phone` class uses `cattr_accessor`, which is `active_support`-specific. Please consider adding the dependency into `.gemspec` or refactor the code. :blush: 

``` ruby
module Phoner
  class Phone
    cattr_accessor :default_country_code
    cattr_accessor :default_area_code  
    cattr_accessor :named_formats
    ....
```
